Semiconductor devices operate by placing a plurality of regions of differently doped, semiconductive material adjacent one another. The purity of the constituent semiconductor materials, critically affects the current carrying characteristics of the device. This is especially true as integrated circuits, made of semiconductor devices, become smaller. In smaller devices, contaminating ions will deplete a greater percentage of device real estate. Thus, contamination is more detrimental to performance as devices get smaller. Therefore, the number of contaminants which adhere to the device during processing must be carefully controlled.
Integrated circuits, where many semiconductor devices are utilized collectively, are formed by depositing semiconducting materials on a substrate arranged such that a desired current flow is realized. Processing is typically done in a clean room, which is a self-contained room with strict regulations about the number of stray particles allowed in the air. However, it is impossible to ensure that a clean room is particle free. Although human operators wear protective clothing and filtering systems function continuously, debris is brought into the clean room each time an operator enters. Additionally, the human body naturally discharges particles, such as dead skin, further adding to clean room contamination. Thus, a clean room contains particles to which the semiconductor can be exposed and thereby become contaminated.
Another threat to the purity of semiconductor devices is inherent in their processing. Many types of semiconductor processing, including pre-diffusion cleaning of the wafer, etching, i.e., removal of material to form the desired layout, and photo-resist removal after etching is completed require the device to be exposed to liquids For example, a transistor in an integrated circuit has many layers of different materials. Fabrication requires an extremely clean substrate on which the dopant is diffused. Thus, the substrate must be cleaned prior to diffusion. Pre-diffusion cleaning requires the wafer to be exposed to liquid which cleans the substrate and liquid which rinses off the cleaner and stray contaminants.
During rinsing, contaminating particles can become attached to the wafer, seriously degrading the quality of the semiconductor device. Typically, the tanks containing liquid baths are exposed to the atmosphere of the clean room. As discussed before, a clean room is never totally uncontaminated. Contaminants fall to the surface of the bath liquids, where they remain until the bath is disturbed. As the wafer moves across the bath/air interface, contaminants that rest on the liquid's surface can stick to the wafer surface. This is especially problematic during removal of the wafer from the final rinse bath since, thereafter, the wafer is not cleaned again.
An additional problem faced in the processing of semiconductor wafers is the fact that as a bath ages, it receives more contaminants. Uniformity throughout batches is extremely important to ensure the reliability of a given semiconductor device. A consumer of semiconductor devices expects that each time a given device is replaced it will function as well as the previous device did. Thus, it is important that each batch of wafers is identical. Since contaminants on the surface of a bath increase with the life of the bath, the first batch of wafers processed can be extremely different than the final batch of wafers processed before the fluid is discarded.
Further problems in semiconductor processing arise due to the handling of the devices. Typically, a device is dipped in a processing bath and then rinsed and moved to another bath. Contaminants, which can adhere to the device, can be generated by the transport machinery. Time spent in the air, between baths, allows contaminants present even in clean rooms to become attached to the device. Additionally, transport of the wafers often leads to breakage. Thus, it is advantageous to minimize wafer movement and, hence, the risk of damage. Furthermore, the machinery required to mechanically move a wafer from one bath to another can be quite expensive.
To eliminate some of the above discussed problems associated with semiconductor processing, several systems have been employed. One, a megasonic cleaning apparatus, is disclosed in U.S. Pat. No. 4,869,278 to Bran. This apparatus uses high frequency energy to agitate a cleaning solution, thereby enabling the agitating liquid to loosen particles on the surfaces of semiconductor wafers. A transducer is aligned beneath a transmitter at the bottom of a bath container. The transducer is electrically excited at its resonant frequency and the transmitter couples the vibrational energy into the bath fluid. The bath liquid vibrates at the frequency output by the transducer. Contaminants are thus shaken loose from the surface of the wafer.
The megasonic process is similar to ultrasound, where sound waves vibrate a bath of liquid to clean objects. However, high frequency oscillation is necessary to ensure that random sonic waves typically generated in liquids oscillating at ultrasound frequencies of approximately 20 to 40 kHz do not form. These sonic waves cause tiny cavities in the bath which implode and damage the wafer. Higher frequency vibration does not generate these types of cavities in vibrating liquid. Thus, megasonic, or high frequency, vibration is preferable in semiconductor processing.
One partial solution to minimize contamination and to accelerate the overall process has been to utilize a dump valve in rinsing tanks wherein a large valve in the bottom of the tank is opened to quickly empty the tank. A further processing or drying cycle can then quickly commence.
Another device which attempts to overcome contamination in processing semiconductor devices is disclosed in U.S. Pat. Nos. 4,633,893; 4,795,497; 4,856,544 and 4,899,767 to McConnell, et al. This device has a chamber wherein wafers are held in a cassette and fluids continuously flow over the faces of the wafers to process them. This allows the wafers to remain stationary as different baths continuously pass over them, and no machinery is necessary to move the wafers from one bath to another. Additionally, a uniform flow of liquid covers the wafer at all times during its processing such that the wafer is not exposed to the atmosphere of the clean room where it can be contaminated. Thus, this device overcomes some of the previously discussed problems associated with contamination and the possibility of damage to the wafer. However, the liquid near the surface of the wafer simply flows across the surface, removing loose contaminants. If a contaminating particle is substantially stuck to the surface of the wafer, this system may not reliably clean contaminants from the wafer. Also, the system is somewhat complicated and costly.
Thus, a further need exists for a device which allows the use of an improved cleaning system to remove particles stuck to the surface of a wafer without necessitating movement of the wafers during cleaning.